The optical communication system performs large-capacity optical transmission by using, as technique of increasing a transmission capacity per optical fiber, wavelength multiplexing communication technique in which multiple optical signals having different wavelengths are transmitted together in one optical fiber. In a wavelength multiplexing communication system, multiple light sources having different wavelengths are used as a laser light source. In a conventional system, since one laser light source is used for an optical signal of one wavelength, the number of laser light sources corresponds to the number of wavelengths to be used (see, for example, Japanese Laid-open Patent Publication No. 2008-227154).
A conventional wavelength multiplexing communication system will be described below with reference to FIG. 13. FIG. 13 is an outline configuration diagram of a conventional wavelength multiplexing communication system. Optical signals 1 to N (wavelengths λ1 to λN) from semiconductor lasers 711 to 71N corresponding to the number N of wavelengths to be used are modulated by optical modulators 721 to 72N, and are multiplexed by an optical multiplexer 73. Then, the multiplexed signal is transmitted via one optical fiber 74.
In contrast, a multi-wavelength laser light source is capable of outputting laser having multiple wavelengths by using one laser. Thus, in a wavelength multiplexing communication system, the system configuration may be simplified by reducing the number of laser light sources to be used (see, for example, Japanese Laid-open Patent Publication No. 2012-191075 or Japanese Laid-open Patent Publication No. 2012-203264).
FIG. 14 is an explanatory diagram of an example of a conventional multi-wavelength laser light source. In a multi-wavelength laser light source 80, a semiconductor optical amplifier 81 is provided between a pair of mirrors 82 and 83, and an etalon 84 is inserted between the semiconductor optical amplifier 81 and the mirror 82. Only light having a desired wavelength among the light having a wide range of wavelengths generated by the semiconductor optical amplifier 81 is selectively transmitted via the etalon 84 so as to oscillate laser light alone with the selectively transmitted wavelength.
As described above, laser light in which multiple wavelengths are mixed is output from a multi-wavelength laser light source. On the other hand, in a wavelength multiplexing communication system, optical signals of respective wavelengths are modulated independently by separate optical modulators. Therefore, in the case of a multi-wavelength laser light source, an optical splitter has to be used so that light is divided and the divided light is input to respective optical modulators on a wavelength basis.
With reference to FIG. 15, a wavelength multiplexing communication system that is usually employed when a multi-wavelength laser light source is used will be described below. FIG. 15 is an outline configuration diagram of a wavelength multiplexing communication system that is usually employed when a multi-wavelength laser light source is used. Laser light having multiple wavelengths from the multi-wavelength laser light source 80 is split by an optical splitter 75, and the split laser light is then modulated by the optical modulators 721 to 724 on a wavelength basis. The modulated optical signals are multiplexed by the optical multiplexer 73, and the multiplexed optical signal is transmitted via one optical fiber 74. Note that, although the case of four wavelengths is illustrated here, the system is not limited to the case of the four-wavelength multiplexing.
When a multi-wavelength laser light source is used in a wavelength multiplexing communication system, however, there is a problem that, while the use of the multi-wavelength laser light source may reduce the number of lasers, an optical splitter has to be added.
Therefore, in a multi-wavelength laser optical source and a wavelength multiplexing communication system, one of the objects is to reduce the number of components of the system and construct a wavelength multiplexing communication system with a simple configuration.